concreting in winter or cold weather is considered as special concrete placement. The process is quite challenging, but it’s not impossible to do. When there is a difference in materials, placing method, properties, or environmental conditions, we treat that concrete as special concrete. According to the American concrete institute, ACI 306 cold weather is defined using the following two conditions
- The average environmental temperature is below 40°F(5°C) for successive 3 days’ time.
- The average environmental temperature should below than 50°F (10°C) during more than half of any 24-hour duration
Risk behind cold weather concreting
If you experience the above conditions at the site, your concreting specification should match with cold-weather concreting requirements. Concreting in winter conditions can cause a defect in many ways. Understand them and prepare your work site beforehand.
- Freezing water will not involve in the hydration process. This will cause un-hydrated cement present in the concrete. As a result of that, the concrete strength will reduce
- Ice occupies 9% more volume than water; the expansion will generate small cracks in the concrete making weak in durability. 50% of durability will lose if concrete freezes below than 25°F (-4 °C) temperature
- When the mix is colder, setting time will increase. When the temperature reduces more than 19.5 °F (-7 °C), the concrete hardening process will stop
- If the freezing rate is high, the concrete more vulnerable to freezing attacks. Fresh concrete freezes before 24 hrs can lose 50% of its 28 days strength
- Exposing the concrete to colder weather during hardening is harmful. More than 68°F (20°C) temperature difference between the inside and outer surface of the concrete can cause thermal cracks.
- Excessive heating will dry out the surface. This can cause shrinkage cracks on concrete.
- Too much curing will saturate the concrete. This will occupy the void structure that is essential to bear the freeze-thaw attack. More than 90% of saturation is harmful
It is vital to maintain general site conditions until the concrete is mature enough for adverse weather conditions. There are few recommendations to attend before, during, and after the cold weather concreting.
Recommendations for concreting in winter conditions
- Check the recent weather reports and predictions. Be sure that you are concreting in cold weather conditions
- Arrange enough materials, forms, testing, and other requirements. Few extra bags of type III cement can be useful
- Use the weather predictions and arrange a schedule for the work. Try to select the best times for the pour, final setting time, and formwork removal.
- Heat the material if necessary, to maintain fresh concrete temperature. Mix the concrete with more than 44.6°F (7°C) temperature. Do not heat material more than 176°F (80°C).
- Insulate concrete pipes and the drums of agitator trucks.
- Never place concrete on the frozen surface. Use heaters to heat-up the platform. It’s better to maintain a temperature between 35.5°F – 41°F (2-5 °C) more than the fresh concrete temperature.
- Try to use wooden forms rather than steel. Wood will insulate the concrete. You can prepare multi-layered insulated forms.
- Arrange wind barriers and plastic blankets before work. Otherwise, you will lose the heat inside the concrete too quickly.
- Calculate the protection time for concrete. Reach at least 3.5 MPa without letting freezing the concrete. Protection time varies with the temperature. Typically 72 hrs with 41°F (5°C) and 46 hrs with 50°F (10°C). Consider the use of accelerators (conforming to ASTM C494/C494M) to gain strength in a short time.
- Typically reducing 50°F (10°C), placing temperature can double the setting time. It is recommended to have a low slump and low water/cement ratio. This reduces the setting time.
- Use air-entraining admixtures, pore structure in the concrete will resist the freeze-thaw attack. Entrapped air will reduce bleeding in concrete.
During & after the concrete
- Maintain records on concrete and environmental temperature.
- Avoid delays in transporting and placing. Waiting will increase the freezing risk.
- Try to maintain temperature more than 50°F (10°C) until you reach the required maturity (use ASTM C1074) for removal of forms.
- Do not let the concrete freeze under any circumstance during the first 24 hrs. Add calculated amount of extra cement to reduce setting time and improve hydration heat
- Use heaters to control the environmental temperature. If combustion heaters are using, arrange the exhausts outside.
- Do not let the surfaces dry or too wet. Maintain adequate, curing conditions.
- Cover the concrete with plastic blankets and limit rapid temperature changes in concrete.
- Do not start finishing works while bleed water is present.
- Remove forms during morning hours. Do not expose the concrete to adverse conditions
The critical factor controlling the successful cold-weather concrete is keeping the concrete warm. You need to make sure the concrete is hardening by not letting it freeze. Use the above list of recommendations and refer ACI 306R-16 for more details.